The Blue Moon Explained

A full Moon
The Moon isn't blue, but the phenomenon of a "Blue Moon" (meaning an extra full Moon in a given period), is very real. NASA
"Once in a blue moon."

Everyone has heard or seen that expression but might not know what it means. It's actually a fairly common saying, but really doesn't reference a blue-colored Moon (our closest neighbor in space). Anyone who steps outside to see the Moon can tell pretty quickly that the Moon's surface is actually a dull grey. In the sunlight, it appears a bright yellow-white color, but it never turns blue. So, what's the big deal with the term "blue moon"? It turns out to be more of a figure of speech than anything else.

Viewing the moon in a telescope.
Near-full moon on November 14, 2016. The full moon gives a wide variety of features to explore with any size telescope or binoculars. Tom Ruen, Wikimedia Commons. 

Decoding a Figure of Speech

The term "blue moon" has an intersting history. Today, it has come to mean "not very often" or "something very rare". The figure of speech itself may have begun with a little-known poem written in 1528, Read me and be not wrothe, For I say no things but truth:

"If they say the moon is blue,
"We must believe that it is true."

The poet was trying to convey the diea that calling the Moon blue was an obvious absurdity, like saying it was made of green cheese or that it has little green men living on its surface. The phrase, “until a blue moon” developed in the 19th century, meaning "never", or at least "extremely unlikely." 

Another Way to Look at the Idea of Blue Moon

"Blue Moon" is more familiar these days as a nickname for an actual astronomical phenomenon. That particular usage first started in 1932 with the Maine Farmer’s Almanac. Its definition included a season with four full Moons rather than the usual three, where the third of four full Moons would be called a "Blue Moon." Since seasons are established by the equinoxes and solstices and not calendar months, it is possible for a year to have twelve full Moons, one each month, yet have one season with four. 

The VLT Observatory in Paranal, Chile.
A setting full moon provides a backdrop for the Very Large Telescope complex in Paranal, Chile. This is one of several high-altitude observatories in South America alone. ESO 

That definition mutated into the one most quoted today when, in 1946, an astronomy article by amateur astronomer James Hugh Pruett misinterpreted the Maine rule to mean two full Moons in one month. This definition now seems to have stuck, despite its error, possibly thanks to being picked up by the Trivial Pursuit game.

Whether we use the newer definition or the one from the Maine Farmer’s Almanac, a blue Moon, while not common, does happen pretty regularly. Observers can expect to see one about seven times in a 19-year period.

Much less common is a double blue Moon (two in one year). That only happens once in the same 19-year period.The last set of double blue Moons happened in 1999. The next ones will happen in 2018.

Can the Moon Appear to Turn Blue?

Normally in the course of a month, the Moon doesn't turn blue itself. But, it can look blue from our vantage point on Earth due to atmospheric effects. 

In 1883, an Indonesian volcano named Krakatoa exploded. Scientists likened the blast to a 100-megaton nuclear bomb. From 600 km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to the very top of Earth's atmosphere and the collection of that ash made the Moon look a bluish color.

Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide, which is the right size to scatter red light, while allowing other colors to pass. White moonlight shining through the clouds emerged blue, and sometimes almost green.

Blue moons persisted for years after the eruption. People also saw lavender suns and, for the first time, noctilucent clouds. Other less-potent volcanic eruptions have caused the Moon to look blue, too. People saw blue moons in 1983, for instance, after the eruption of the El Chichón volcano in Mexico. There were also reports of blue moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.

It's fairly easy to see a Blue Moon that isn't a colorful metaphor. In astronomical terms, it's almost guaranteed observers will see one if they know when to look. Searching out a moon that acually appears blue, well, that's something that is possibly more rare than the fourth full moon in a season. It takes a volcanic eruption or a forest fire to affect the atmosphere enough to make the Moon appear colorful through all the haze.

Key Takeaways

  • A Blue Moon is NOT a moon that's blue.
  • The best description of the term "Blue Moon" is that it's a figure of speech now used to refer to an extra full moon in any season (or in the same month).
  • While the Moon itself doesn't ever turn blue, it can appear blue if there is a lot of ash in Earth's atmosphere due to a volcanic eruption or other atmospheric effects.


  • “How Rare Is a Blue Moon?”,
  • VolcanoCafe,

Edited by Carolyn Collins Petersen.

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Your Citation
Greene, Nick. "The Blue Moon Explained." ThoughtCo, Aug. 27, 2020, Greene, Nick. (2020, August 27). The Blue Moon Explained. Retrieved from Greene, Nick. "The Blue Moon Explained." ThoughtCo. (accessed March 28, 2023).